Abstract
Wave tank testing of wave energy converters (WECs) is generally undertaken for one of two reasons: numerical model identification/validation or performance evaluation. In the case of performance evaluation, some aspect(s) of WEC performance, such as power absorption, power generation, or loading is assessed using an experimental device. With a large number of performance assessment experiments being conducted on various devices, it is key that the design of experiments be such as to maximize cost efficiency. Advanced control of the power take-off (PTO) in a WEC has shown significant promise for increasing wave energy absorption in simulation [1, 2]. The tuning of control strategies, which should be performed numerically to the extent possible, adds another factor to assessing WEC performance in wave tank testing.
A 1/17th scale WEC device has been designed for tests concerned with the study of WEC modeling and control [3, 4]. Figure 1 and Table 1 show a diagram of the WEC device and its relevant physical parameters. A detailed description of the basin and how the WEC device was tested within the basin (i.e., installation, location, mounting) is given in [3]. The WEC is capable of moving in three degrees of freedom (heave, pitch, and surge) to allow for full motion in a single plane. Each degree of freedom is actuated independently. In addition, a series of large springs are used to provide a restoring force in surge. The inclusion of the springs in the design was chosen to improve the safety of the system. Since there is no hydrostatic restoring reaction in surge, an instability or error in the actuator system (or even a loss of power during a wave tests) could otherwise cause damage to the system.
For this study, we consider a JONSWAP sea state with Hs = 0.127 m, Tp = 3.5 s, and γ = 3.3. First, the concept of short repeat period wave trains for performance assessment is considered. Next, a matrix of proportional and integral control gains (i.e., P and PI controllers, see, e.g., [5]) are evaluated in the wave tank.